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The insulator to metal transition ͑IMT͒ induced by the application of an electric field in doped manganites is investigated theoretically. Starting from the double-exchange mechanism with the long-range Coulomb interaction included, we find that the electric field may suppress the charge ordering and drive the system from the antiferromagnetic and charge-ordered state with an energy gap at the Fermi level to the ferromagnetic and gapless state, resulting in the IMT. A numerical simulation is performed for manganite films with intrinsic inhomogeneities, and an important impact of the inhomogeneities on this electric-field-induced transition is obtained. Our results can naturally account for the recently observed electric-filed-induced IMT phenomenon in manganites.The carrier-doped manganites with perovskite structure, R 1Ϫx A x MnO 3 ͑where R and A are rare and alkaline-earth ions, respectively͒ have attracted much attention maily due to fruitful physical properties that they exhibit. One particular and interesting property is the insulator to metal transition ͑IMT͒ observed in some narrow bandwidth manganites, which can be triggered by the variation of the doping, 1 temperature, 2 and the application of some external forces, such as a magnetic field, 3 a higher pressure, 4 an exposure to x rays 5 or visible light, 6 and even an electric field. 7-12 In the presence of magnetic field, the observed ''colossal magnetoresistance'' phenomenon can be understood qualitatively in terms of the double-exchange ͑DE͒ mechanism: the strong on-site Hund's coupling aligns the spin of the extra e g electron on each Mn 3ϩ parallel to the corelike t 2g spins so that the transfer of this electron to the neighboring Mn 4ϩ site depends on the relative configuration of the two neighboring core spins, which can be affected by the magnetic field. Although the DE model provides a simple but natural explanation for the IMT induced by the magnetic field ͑or by the pressure͒ via enhancing the effective electron hopping, whether it can also account for the IMT induced by an electric field ͑or light͒ is not so clear as there is no direct connection between the latter and the magnetic configuration of the system. Furthermore, unlike the magnetic field and high pressure which cause the whole sample to undergo the phase transition, it was reported experimentally that the application of the electric field results in an IMT, where a nucleation of metallic patches that form one or more filamentary metallic paths may occur. 6,7 According to Ref. 7 the ground state of Pr 1Ϫx Ca x MnO 3 with xϭ0.3-0.5 is a charge-ordered antiferromagnetic ͑AF͒ insulator with the charge ordering ͑CO͒ having the pattern similar to that at half doping (xϭ0.5). The application of a static electric field triggers the collapse of the low temperature, electrically insulating CO state to a metallic ferromagnetic ͑FM͒ state. After being switched into the much lower-resistance state from the higher-resistance state at a threshold voltage V th in the increasing-voltage scan, the ...

Using a tunneling Hamiltonian including spin-flip effect, we derive general expressions for spin-polarized tunneling conductances as well as magnetoresistance (MR) in ferromagnetic/ insulator/ferromagnetic junctions. The spin-dependent density of states due to exchange splitting is responsible for the large negative MR, and the spin-flip tunneling is found to diminish the magnitude of MR. Qualitative agreement is obtained between our theoretical results and recent experimental data.

The photoluminescence spectra as well as their temperature dependence of the tellurium and nitrogen (Te-N) codoped ZnO films have been investigated in detail. Explicit evidences of the emissions related to two acceptors [A1: the NO-Zn-Te subunits and A2: the conventional N ions substituting on oxygen sites (NO)] have been found. The acceptor activation energy level of the A1 (∼118–124 meV) is much shallower than that of the A2 (∼224–225 meV) indicating that the A1 should be mainly responsible for the room-temperature p-type nature of the codoped samples. Meanwhile, the acceptor activation energy level of A1 shows a slight decrease (∼6 meV) as the Te atomic concentration increases in the codoped samples implying that the actual form of the A1 may be a mixture of the NO-Zn-nTe (n = 1, 2, 3, 4). More incorporation of the Te ions into N-doped ZnO films not only makes the acceptor energy level shallower but also improves the crystalline quality and results in the efficiently suppressed native donorlike defects. The optical properties accord well with the crystalline and electrical ones indicating that the Te-N codoping technique is a potentially feasible route to get controllable p-type ZnO materials.

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